SILICONE LASTORS FOR HVDC

Description

[0001] The present invention relates to a cross-linked silicone elastomer, a method for its production and its use in high voltage direct current (HVDC) applications. State of the art

[0002] Long-distance power transmission using HVDC can be significantly more cost-effective than using high-voltage alternating current (HVAC) systems due to lower electrical losses. However, underground HVDC transmission requires a high number of cable connections, typically every 1 to 2 km.

[0003] However, the insulating materials used in high-voltage alternating current systems are mostly not suitable for use in HVDC systems, as the electrical stresses for alternating and direct current conditions can be significantly different.

[0004] The local distribution of the electric field in HVDC applications is determined by the specific volume resistivity of the electrical insulating materials used. Therefore, in the prior art, EPDM (ethylene propylene diene monomer rubber) is predominantly used in these cable connections, as its resistivity is lower than that of polyolefin-based insulating materials for the cables.

[0005] Due to its high hardness and the large number of fillers (impurities) used, EPDM often exhibits adverse behavior during installation and operation.

[0006] Silicone elastomers are not currently used for HVDC applications because their resistance is too high compared to the cable insulation material.

[0007] In the prior art, electrically conductive fillers (e.g., metal powders, metal flakes, carbon blacks, or carbon nanotubes) are therefore used to adjust the resistance of the cured silicone elastomer. Such fillers can pose further problems because, due to the very small quantities involved, it is almost impossible to distribute these fillers uniformly throughout the material during the mixing process in order to achieve consistent electrical properties within the cured silicone elastomer. Furthermore, they lead to a deterioration of the physical properties and a reduced dielectric strength of the cured silicone elastomer. WO2021195038A1 discloses silicone compositions containing silica fillers, some of which have been surface-treated with fluorinated hydrophobic agents. Here, the high raw material costs are a significant disadvantage.

[0008] US2020350094 A1 proposes crosslinked silicone elastomers with electrically conductive fillers for adjusting the volume resistance of a crosslinked silicone elastomer to the volume resistance of a cable insulation.

[0009] In summary, the following disadvantages of the systems known in the prior art are evident: The disadvantage of filled systems consists firstly in the reproducibility (e.g. steep drop in resistance in the area of ​​the percolation threshold), and secondly in their possible anisotropic effect (metal oxides on platelet-like support systems) and their dependence on moisture.

[0010] Fluorinated systems are generally very expensive, and halogenated polymers should generally be avoided as much as possible for environmental reasons.

[0011] Mixtures based on EPDM are hard, difficult to process and assemble.

[0012] Therefore, there is a great need for silicone compositions for the production of silicone elastomers for HVDC applications that do not exhibit the aforementioned disadvantages of the state of the art.

[0013] Surprisingly, it was found that the present cross-linked silicone elastomers according to the invention exhibit the necessary reduction in electrical resistance permanently.

[0014] Subject matter of the present invention These are therefore cross-linked silicone elastomers whose volume resistance has been adjusted to match the volume resistance of an adjacent cable insulation. wherein this volume resistance is determined on a 0.5 mm thick cross-linked silicone elastomer, in a heated protective ring arrangement with an electric field strength of 1 kV / mm, according to standard IEC 62631-3-1 and meets the following value after application of the test voltage: after 10,000 minutes: < 1.0 10 16 < Ohm*cm, obtainable by cross-linking a base mass containing: (A) 50 to 99 wt.% of at least one diorganopolysiloxane with at least 2 cross-linking groups per molecule, (B) 0.5 to 5 wt.% of at least one peroxide, (C) 0 to 50 wt.% of at least one reinforcing filler, and (X) NO conductive or semiconducting additives, wherein the quantity of all components is based on 100 wt.% add, wherein this base mass is applied to a substrate or filled into a mold, in a first step crosslinking is carried out by heating to at least the temperature of the 10 h-HWT (= 10 hour half-life) of the peroxide (B), wherein the heating duration corresponds to at least 0.2 of a half-life of the peroxide (B) at the selected crosslinking temperature, in a second step tempering is carried out above the temperature of the 10 h-HWT of the peroxide (B), wherein the tempering duration corresponds to at least one half-life of the peroxide (B) at the selected tempering temperature.

[0015] To avoid making the description of the present invention too lengthy, only the preferred embodiments of the individual features are listed below. However, the reader skilled in the art should explicitly understand this type of disclosure to mean that every combination of different preferred states is explicitly disclosed and explicitly desired.

[0016] The volume resistance of these cross-linked silicone elastomers according to the invention is After 1 minute: < 1.0 x 10^15 < Ohm*cm; preferably < 8.0 x 10^14 < Ohm*cm; particularly preferred < 5.0 x 10^14 < Ohm*cm; after 15 minutes: < 3.0 x 10^15 < Ohm*cm; preferably < 2.0 x 10^15 < Ohm*cm; particularly preferred < 8.0 x 10^14 < Ohm*cm; after 10,000 minutes: < 1.0 x 10^16 < Ohm*cm; preferably < 8.0 x 10^15 < Ohm*cm; particularly preferred < 6.0 x 10^15 < Ohm*cm.

[0017] Energy sources used for crosslinking and tempering by heating are preferably ovens, e.g., convection drying ovens, heating channels, heated rollers, heated plates, heated molds or infrared heat radiation.

[0018] It has been shown that the volume resistivity of the cured silicone elastomers according to the invention can be reliably adjusted solely by the amount of peroxide (B) according to the invention in combination with the crosslinking and tempering conditions according to the invention, even without the use of conductive or semiconducting additives (X). Soft silicone elastomers can be produced whose advantageous properties (resistance to electrical aging, gas permeability, translucency, elasticity over wide temperature ranges) have already proven superior to other materials in AC applications. This possibility according to the invention, which now also allows the volume resistivity to be reliably adjusted, now makes it possible to utilize these advantages for DC applications as well. Measurement methods for measuring through-resistance:

[0019] The measurement is performed on a 0.5 mm thick cross-linked silicone elastomer, in a heated protective ring arrangement with an electric field strength of 1 kV / mm, according to the standard IEC 62631-3-1 "Guidelines for the determination of dielectric and resistive properties of solid insulating materials - Part 3-1: Determination of resistive properties (DC Methods) - Volume resistance and volume resistivity, general method"

[0020] The measuring instruments used are protective ring measuring cells from Tettex Instruments "Solid Test Cell 2914" with megohmmeter "Eaton Sefelec 1500-M" or Sefelec M1501 P.

[0021] The through-resistance was measured and the specific through-resistance was calculated from it.

[0022] Preferably, the heating time of the crosslinking in the first step is carried out for at least one half-life of the peroxide (B) at the selected crosslinking temperature, particularly preferably for at least two half-lifes of the peroxide (B) at the selected crosslinking temperature.

[0023] In a preferred embodiment, the crosslinking in the first step takes place at a temperature of at least the 10 h-HWT (= 10 hour half-value temperature) up to a maximum of the 1 min-HWT (= 1 minute half-value temperature) of the peroxide (B) used, particularly preferably at a temperature of at least the 10 h-HWT up to a maximum of 10°C below the 1 min-HWT of the peroxide (B) used. Component (A)

[0024] The component (A) of the composition according to the invention is a diorganopolysiloxane or a mixture of diorganopolysiloxanes of the general formula (1): R 1< a R 2< 3 SiO (4-ab) / 2 (1)

[0025] R1< is a substituted or unsubstituted monovalent hydrocarbon residue that does not contain any aliphatic unsaturated groups. R2< is a substituted or unsubstituted monovalent hydrocarbon residue that is aliphatic unsaturated.

[0026] The indices a and b are positive numbers in the range 1 ≤ a < 3, 0 ≤ b ≤ 1 and 1 < a + b ≤ 3.

[0027] In a preferred embodiment, each molecule contains on average at least two unsaturated groups R 2< bonded to silicon atoms.

[0028] In particular, R 1< means a monovalent, SiC-bonded, optionally substituted hydrocarbon residue free of aliphatic carbon-carbon multiple bonds with 1 to 18 carbon atoms.

[0029] Examples of residues R 1< are alkyl residues, such as methyl, ethyl, n-propyl, iso-propyl, 1-n-butyl, 2-n-butyl, iso-butyl, tert-butyl, n-pentyl, iso-pentyl, neo-pentyl, tert-pentyl residues, hexyl residues, such as n-hexyl residue, heptyl residues, such as n-heptyl residue, octyl residues, such as n-octyl residue and iso-octyl residues, such as 2,2,4-trimethylpentyl residue, nonyl residues, such as n-nonyl residue, decyl residues, such as n-decyl residue, dodecyl residues, such as n-dodecyl residue, and octadecyl residues, such as n-octadecyl residue; Cycloalkyl groups, such as cyclopentyl, cyclohexyl, cycloheptyl and methylcyclohexyl groups; aryl groups, such as phenyl, naphthyl, anthryl and phenanthryl groups; alkaryl groups, such as o-, m-, p-tolyl groups, xylyl groups and ethylphenyl groups; and aralkyl groups, such as benzyl, α- and β-phenylethyl groups.

[0030] Examples of substituted residues R 1< are haloalkyl residues, such as the 3,3,3-trifluoro-n-propyl residue, the 2,2,2,2',2',2'-hexafluoroisopropyl residue, the heptafluoroisopropyl residue and haloaryl residues, such as the o-, m- and p-chlorophenyl residue, as well as all residues mentioned above for R, which may preferably be substituted with mercapto groups, epoxy functional groups, carboxyl groups, keto groups, enamine groups, amino groups, aminoethylamino groups, iso-cyanato groups, aryloxy groups, acryloxy groups, methacryloxy groups, hydroxy groups and halogen groups.

[0031] Preferably, the residue R 1< is a monovalent hydrocarbon residue with 1 to 6 carbon atoms, with the methyl residue being particularly preferred.

[0032] R 2< in particular means a monovalent, SiC-bonded hydrocarbon residue with an aliphatic carbon-carbon multiple bond.

[0033] Examples of residues R 2< are alkenyl residues, such as the vinyl, 5-hexenyl, cyclohexenyl, 1-propenyl, allyl, 3-butenyl and 4-pentenyl residues, and alkynyl residues, such as the ethynyl, propargyl and 1-propynyl residues.

[0034] Preferably, the residue R 2< is an alkenyl residue, with the vinyl residue being particularly preferred.

[0035] In a preferred embodiment, R1< is a methyl group and R2< is a vinyl group. The structure of the diorganopolysiloxanes (A) can be linear or branched, with a linear structure being preferred. The viscosity of the diorganopolysiloxanes (A) at 25°C (determined according to DIN 53018) is between 1,000 mPa·s and 50,000,000 mPa·s. In a preferred embodiment, the viscosity of the diorganopolysiloxanes (A) is between 500,000 and 40,000,000 mPa·s, and even more preferably between 2,000,000 and 30,000,000 mPa·s, and thus in the range of polysiloxanes commonly used in high-temperature curing rubbers (HTV).

[0036] In another embodiment, the viscosity of the diorganopolysiloxanes (A) at 25°C (determined according to DIN 53018) is preferably between 1,000 mPa·s and 100,000 mPa·s, more preferably between 5,000 and 50,000 mPa·s. Polysiloxanes in this viscosity range are commonly used for liquid silicone rubber (LSR).

[0037] The diorganopolysiloxanes (A) can be, for example, vinyl-terminated polydimethylsiloxanes, vinyl-terminated polydimethyl-polymethylvinylsiloxanes or trimethylsilyl-terminated polydimethyl-polymethylvinylsiloxanes.

[0038] Component (A) can consist of a single diorganopolysiloxane or of mixtures of two or more diorganopolysiloxanes. (A) is used in amounts of 50 wt.% to 99 wt.%, preferably 55 wt.% to 85 wt.%, in particular 60 wt.% to 80 wt.%. Component (B)

[0039] Peroxides, which serve as sources of free radicals, are used as crosslinking agents. They are selected from the group of dialkyl peroxides, diaryl peroxides, alkylaryl peroxides, aralkyl peroxides, and hydroperoxides. Component (B) can be a single peroxide or hydroperoxide, or a combination of different peroxides or peroxides with hydroperoxides.

[0040] Examples of organic peroxides include acyl peroxides, such as dibenzoyl peroxide, bis-(4-chlorobenzoyl) peroxide, bis-(2,4-dichlorobenzoyl) peroxide, and bis-(4-methylbenzoyl) peroxide; alkyl peroxides and aryl peroxides, such as di-tert-butyl peroxide, 2,5-bis-(tert-butylperoxy)-2,5-dimethylhexane, dicumyl peroxide, and 1,3-bis-(tert-butylperoxy-isopropyl)benzene; and perketals, such as 1,1-bis-(tert-butylperoxy)-3,3,5-trimethylcyclohexane. Peresters such as diacetyl peroxydicarbonate, tert-butyl perbenzoate, tert-butyl peroxy-isopropyl carbonate, tert-butyl peroxy-isonona-noate, dicyclohexyl peroxydicarbonate and 2,5-dimethyl-hexane-2,5-diperbenzoate.

[0041] It is known in the prior art that peroxides can be distinguished as vinyl-specific and non-vinyl-specific peroxides. See, for example, the textbook SILICONES by Pachaly et al., WILEY-CH, ISBN-10: 3-527-30770-2, ISBN-13: 978-3527-30770-8; pages 41 ff.

[0042] (B) is used in amounts of 0.5 to 5 wt.%, preferably 1 to 4 wt.%, and particularly preferably 1.5 to 3 wt.%. Peroxides have characteristic parameters: The half-life (TWZ):

[0043] The half-life (HWZ) of a peroxide at a defined temperature indicates the time after which half of the peroxide quantity has decomposed.

[0044] Half-life data can be found in the literature and are also provided by peroxide manufacturers. Values ​​between individual data points can be extrapolated using Arrhenius kinetics. 10 h-HWT:

[0045] The 10-hour half-life temperature is the temperature at which half of the amount of peroxide has decomposed within 10 hours. 1 min HWT:

[0046] The 1-minute half-value temperature is the temperature at which half of the amount of peroxide has decomposed within one minute.

[0047] In a preferred embodiment, component (A) is a vinyl-containing diorganopolysiloxane and component (B) is a vinyl-specific peroxide. Reinforcing fillers (C)

[0048] Reinforcing fillers (C) that can be used are pyrogenic or precipitated silicas with BET surface areas of at least 50 m² / g.

[0049] The aforementioned actively reinforcing silica fillers (C) can have a hydrophilic character or be made hydrophobic by known methods.

[0050] Precipitated and pyrogenic silicas, as well as mixtures thereof, are preferred. Pyrogenic silica surface-treated with a silylating agent is particularly preferred. Methods for hydrophobization have long been known to those skilled in the art. The hydrophobization of the silica can be carried out either before incorporation into the polyorganosiloxane or in the presence of a polyorganosiloxane using an in-situ method. Both methods can be performed in batch or continuous processes. All hydrophobizing agents known to those skilled in the art can be used as silylating agents. These are preferably silazanes, in particular hexamethyldisilazane and / or 1,3-divinyl-1,1,3,3-tetramethyldisilazane, and / or polysilazanes, with water also being an additional component.In addition, other silylating agents, such as SiOH and / or SiCl and / or alkoxy-functional silanes or siloxanes, can also be used as hydrophobizing agents. Likewise, cyclic, linear, or branched non-functional organosiloxanes, such as octamethylcyclotetrasiloxane or polydimethylsiloxane, can be used individually or in combination with silazanes as silylating agents. To accelerate the hydrophobizing process, the addition of catalytically active additives, such as hydroxides, is also possible. Hydrophobizing can be carried out in a single step using one or more hydrophobizing agents, or in multiple steps using one or more hydrophobizing agents.

[0051] Precipitated or pyrogenic silicas are preferred. A silica with a specific surface area according to BET of 80-400 m² / g is particularly preferred, especially 100-400 m² / g.

[0052] Actively reinforcing silica fillers (C) can be used individually or as mixtures.

[0053] The content of reinforcing filler (C) is in the range of 0 to 50 wt.%, preferably 15 to 45 wt.%, preferably 20 to 40 wt.%. Other components (D)

[0054] Other components are well known to those skilled in the art and can be used in the compositions according to the invention. Non-limiting examples include non-reinforcing fillers, plasticizers, adhesion promoters, soluble dyes, inorganic and organic pigments, solvents, fungicides, perfumes, dispersing agents, rheological additives, corrosion inhibitors, oxidation inhibitors, light stabilizers, heat stabilizers, and flame retardants. Component (X)

[0055] The base material according to the invention contains NO conductive or semiconducting additives (X). "NO" means that such additives may be present up to the quantity typically found in impurities. Such additives have long been known to those skilled in the art. Examples include carbon black, metals, metal oxides, and semiconductors (such as SiC, Si) in the form of nanoparticles.

[0056] Another object of the present invention is the method for producing the cross-linked silicone elastomers according to the invention,whose through-resistance is adjusted to the through-resistance of an adjacent cable insulation, wherein this volume resistance is determined on a 0.5 mm thick cross-linked silicone elastomer, in a heated protective ring arrangement with an electric field strength of 1 kV / mm, according to standard IEC 62631-3-1 and meets the following value after application of the test voltage: after 10,000 minutes: < 1.0 1016 Ohm*cm, obtainable by cross-linking a base mass containing: (A) 50 to 99 wt.% of at least one diorganopolysiloxane with at least 2 cross-linkable groups per molecule, (B) 0.5 to 5 wt.% of at least one peroxide, (C) 0 to 50 wt.% of at least one reinforcing filler, and (X) NO conductive or semiconducting additives, wherein the amount of all components is limited to 100 wt.% add, wherein this base mass is applied to a substrate or filled into a mold, in a first step crosslinking is carried out by heating to at least the temperature of the 10 h-HWT (= 10-hour half-life) of the peroxide (B), wherein the heating duration corresponds to at least 0.2 of a half-life of the peroxide (B) at the selected crosslinking temperature, in a second step tempering is carried out above the temperature of the 10 h-HWT of the peroxide (B), wherein the tempering duration corresponds to at least one half-life of the peroxide (B) at the selected tempering temperature.

[0057] Another aspect of the present invention is the use of cross-linked silicone elastomers for Insulation applications, especially for HVDC applications such as HVDC assemblies. Examples

[0058] The following examples describe the fundamental feasibility of the present invention, without, however, limiting it to the contents disclosed therein.

[0059] In the following examples, all parts and percentages refer to weight unless otherwise stated. Unless otherwise stated, the following examples are performed at atmospheric pressure (approximately 1000 hPa) and at room temperature (approximately 20°C), or at the temperature reached when the reactants are combined at room temperature without additional heating or cooling.

[0060] The composition according to the invention can be produced by simply mixing the components in a mixing unit commonly used for silicone rubber compositions (beam stirrer, paddle stirrer, kneader, extruder, two-roll mill).

[0061] In the following and in the tables, it means: E13 = 10¹³ < E14 = 10¹⁴ < E15 = 10¹⁵ < E16 = 10¹⁶ < HWZ = Half-life Peroxides = crosslinking agents

[0062] Networker 1: Crosslinker 1 is dicumyl peroxide with the following properties: 10 h-HWT: 111 °C 1 min-HWT: 168 °C Half-life at 120 °C: ~ 5.3 hours Half-life at 130 °C: ~ 1.6 hours Half-life at 140 °C: ~ 0.5 hours Half-life at 165 °C: ~ 1.5 minutes

[0063] Networker 2: Crosslinker 2 is a 50% paste of 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane in silicone oil with the following properties: 10 h HWT: 116 °C 1 min HWT: 176 °C

[0064] Networker 3: Crosslinker 3 is a 50% paste of bis(4-methylbenzoyl)peroxide in silicone oil. 10 h HWT: 70 °C 1 min HWT: 130 °C

[0065] Networker 4: Crosslinker 4 is a 50% bis-(2,4-dichlorobenzoyl) peroxide paste with the following properties: 10 h HWT: 51 °C 1 min HWT: 119 °C Basic mass 1:

[0066] In a laboratory kneader, 750 g of a vinyldimethylsiloxy-terminated polydimethylsiloxane (PDMS) with a viscosity of 20,000 mPas (25°C) were placed, heated to 150°C, and mixed with 550 g of a hydrophobic pyrogenic silica with a specific surface area (according to BET) of 300 m² / g and a carbon content of 3.9 wt%. This resulted in a highly viscous mass, which was then diluted with 300 g of a vinyldimethylsiloxy-terminated polydimethylsiloxane with a viscosity of 20,000 mPas (25°C). By kneading under vacuum (10 mbar) at 150°C, volatile components were removed within one hour. Basic mass 2:

[0067] In a kneader, 100 parts of a dimethylvinylsilyloxy-terminated dimethylsiloxane-methylvinylsiloxane copolymer, containing 99.94 mol% dimethylsiloxy units and 0.06 mol% methylvinylsiloxy units and exhibiting a degree of polymerization of approximately 6000 siloxy units, were mixed with 41 parts of silica with a surface area, measured according to the BET method, of 300 m2 / g and 7 parts of a dimethylhydroxysiloxy-terminated dimethylsiloxane oligomer with a viscosity of 40 mPa·s until homogeneity was achieved and heated to 170°C for two hours. Example 1 (not according to the invention):

[0068] To 80.0 g of the base compound 1, 18.7 g of vinyldimethylsiloxy-terminated polydimethylsiloxane with a viscosity of 20,000 mPa·s (25°C), 0.1 g of ethynylcyclohexanol, 1.9 g of a copolymer of dimethylsiloxy, methylhydrogensiloxy, and trimethylsiloxy units with a viscosity of 300 mPa·s at 25°C and a SiH content of 0.47%, and 0.1 g of a solution containing a platinum-synthetic divinyltetramethyldisiloxane complex and 1 wt% platinum were added at 25°C. The mixture was homogeneously blended with a paddle stirrer and then degassed in a desiccator (10 min at approximately 10 mbar).

[0069] The silicone compound produced in this way was then cured in a hydraulic press according to the temperature and time specified in the table. The demolded, 0.5 mm thick silicone elastomer sheets were then annealed in a convection oven according to the conditions specified in the table. The volume resistivity was then determined using the described method. Table 1: Processing conditions and data on the contact resistance for example 1 a) and b) Example 1 a) Example 1 b) Networking 5 min, 165 °C 30 min, 100 °C Tempering 4 hours, 200 °C 24 h, 120 °C Contact resistance after 1 min in ohms / cm 2,8 E15 3,3 E15 Contact resistance after 15 min in ohms / cm 6,0 E15 5,7 E15 Contact resistance after 10000 min in ohms / cm 4,1 E16 3,6 E16 Example 2

[0070] To 80.0 g of the base mass 1, 20.0 g of vinyldimethylsiloxy-terminated polydimethylsiloxane with a viscosity of 20,000 mPa·s (25°C) and the amount of peroxide specified in Table 2 were added. The mixture was homogeneously blended using a paddle stirrer and then degassed in a desiccator (10 min at approximately 10 mbar).

[0071] The silicone compound produced in this way was then cured in a hydraulic press according to the temperature and time specified in Table 2. The demolded, 0.5 mm thick silicone elastomer sheets were then annealed in a convection oven according to the conditions specified in the table. The volumetric resistance was then determined using the described method. Table 2: Peroxide quantity, processing conditions and volume resistance for example 2 a) to 2c) Example 2a Example 2b Example 2c Networker 1 0,6 g 2,0 g 3,0 g Networking 15 min 165 °C 15 min 165 °C 15 min 165 °C Tempering 24 h 120 °C 24 h 120 °C 24 h 120 °C Contact resistance after 1 min in ohms / cm 5,3 E14 1,1 E14 4,5 E13 Contact resistance after 15 min in ohms / cm 1,1 E15 2,6 E14 1,2 E14 Contact resistance after 10000 min in ohms / cm 5,5 E15 1,3 E15 6,8 E14 Example 3

[0072] To 80.0 g of the base mass 1, 20.0 g of vinyldimethylsiloxy-terminated polydimethylsiloxane with a viscosity of 20,000 mPa·s (25°C) and the amount of peroxide specified in Table 3 were added. The mixture was homogeneously blended using a paddle stirrer and then degassed in a desiccator (10 min at approximately 10 mbar).

[0073] The silicone compound produced in this way was then cured in a hydraulic press according to the temperature and time specified in Table 3. The demolded, 0.5 mm thick silicone elastomer sheets were then annealed in a convection oven according to the conditions specified in the table. The volumetric resistance was then determined using the described method. Table 3: Type and quantity of peroxide, processing conditions and volume resistance (in ohms / cm) for example 3 a) to 3 c) Example 3a Example 3b Example 3c Networker 1 2,0 g - - Networker 2 - 4.0 g (equivalent to 2.0 g of pure peroxide) - Networker 3 - - 2.0 g (equivalent to 1.0 g pure peroxide) Networking 16 h 130 °C 16 h 130 °C 16 h 130 °C Tempering 24 h 120 °C 24 h 120 °C 24 h 120 °C Contact resistance after 1 min in ohms / cm 5,4 E13 7,4 E13 7,2 E14 Contact resistance after 15 min in ohms / cm 1,5 E14 1,3 E14 1,6 E15 Contact resistance after 10000 min in ohms / cm 8,2 E14 7,2 E14 8,1 E15 Example 4

[0074] The components listed in Table 4 were homogeneously mixed using a paddle stirrer and then degassed in a desiccator (10 min at approximately 10 mbar).

[0075] The silicone compound produced in this way was then cured in a hydraulic press according to the temperature and time specified in Table 4. The demolded, 0.5 mm thick silicone elastomer sheets were then annealed in a convection oven according to the conditions specified in Table 4. The volume resistivity was then determined. Table 4: Composition, processing conditions and volume resistivity (in ohms / cm) for example 4 a) to 4 d) Example 4a Example 4b Example 4c Example 4d Basic mass 1 100,0 g 50,0 g 0 80,0 g Vinyl-terminated PDMS; viscosity 20,000 mPa·s 0 50,0 g 100,0 g Trimethylsilyl-terminated PDMS with a viscosity of 100 mPa·s (25°C) 0 20,0 g Networker 1 2,0 g 2,0 g 2,0 g Networker 2 4,0 g Networking 15 min 165 °C 15 min 165 °C 15 min 165 °C 16 h 130 °C Tempering 24 h 140 °C 4 h 200 °C 24 h 140 °C 24 h 120 °C Contact resistance after 1 min in ohms / cm 7,0 E13 4,0 E14 6,8 E14 3,0 E13 Contact resistance after 15 min in ohms / cm 1,8 E14 8,2 E14 1,2 E15 8,7 E13 Contact resistance after 10000 min in ohms / cm 1,0 E15 4,1 E15 2,8 E15 6,8 E14 Example 5

[0076] A mixture of 100 g of the base compound 2 and 4.0 g of crosslinker 2 was prepared on a roller. The silicone compound produced in this way was then crosslinked in a hydraulic press at 165 °C for 15 minutes. The demolded, 0.5 mm thick silicone elastomer films were then annealed for 4 hours at 200 °C in a convection oven. The volume resistivity was then determined according to the described method. Example 6

[0077] A mixture of 100 g of a polydimethylsiloxane with a degree of polymerization of approximately 6000 siloxy units and 1.5 g of crosslinker 4 was prepared on a roller. The silicone compound produced in this way was then crosslinked in a hydraulic press at 165 °C for 15 minutes. The demolded, 0.5 mm thick silicone elastomer films were then annealed for 8 hours at 200 °C in a convection oven. The volume resistivity was then determined according to the described method. Table 5: Contact resistance for Example 5 and Example 6 Example 5 Example 6 Contact resistance after 1 min in ohms / cm 3,1 E14 4,7 E14 Contact resistance after 15 min in ohms / cm 6,6 E14 8,6 E14 Contact resistance after 10000 min in ohms / cm 3,4 E15 3,0 E15 Example 7

[0078] To 80.0 g of the base compound 1, 20.0 g of vinyldimethylsiloxy-terminated polydimethylsiloxane with a viscosity of 20,000 mPa·s (25°C) and 2.0 g of crosslinker 1 were added at 25 °C. The mixture was homogeneously blended using a paddle stirrer and then degassed in a desiccator (10 min at approximately 10 mbar).

[0079] The silicone compound produced in this way was then cured in a hydraulic press according to the temperature and time specified in the table. The demolded, 0.5 mm thick silicone elastomer sheets were then annealed in a convection oven according to the conditions specified in the table. The volume resistivity was then determined using the described method. Table 6: Processing conditions and through-resistances for example 7 a) - d) and 7e) - g) Example 7a Example 7b Example 7c Example 7d Networking 130 °C for 30 minutes (~ 0.3 times half-life) 130 °C for 2 hours (1.25 times the half-life) 130 °C for 4 hours (2.5 times half-life) 130 °C 16 hours (10 times half-life) Tempering 24 h 120 °C 24 h 120 °C 24 h 120 °C 24 h 120 °C Contact resistance after 1 min in ohms / cm 1,0 E15 2,0 E14 7,2 E13 7,2 E13 Contact resistance after 15 min in ohms / cm 1,9 E15 7,1 E14 2,5 E14 1,4 E14 Contact resistance after 10000 min in ohms / cm 8,2 E15 4,9 E15 2,1 E15 9,0 E14 Example 7e Example 7f Example 7 Networking 140 °C 30 minutes (~ 1 time half-life) 140 °C 1 hour (2 times half-life) 140 °C for 2 hours (4 times half-life) Tempering 24 h 120 °C 24 h 120 °C 24 h 120 °C Contact resistance after 1 min in ohms / cm 4,3 E14 1,3 E14 4,5 E13 Contact resistance after 15 min in ohms / cm 9,1 E14 2,9 E14 1,6 E14 Contact resistance after 10000 min in ohms / cm 6,1 E15 2,0 E15 1,1 E15 Example 8

[0080] The test plate from Example 7 c) was tightly wrapped in aluminum foil after the volume resistance was determined and stored at 80 °C. After the storage time specified in the table, the test plate was removed from the oven and the volume resistance value (15-minute measurement) was determined. The plate was then re-wrapped in aluminum foil and stored at 80 °C. Table 7: Storage duration and volume resistance for example 8. Storage duration (hours) Contact resistance after 15 min in ohms / cm 0 3,7 E14 168 3,5 E14 340 3,2 E14 500 3,6 E14 1430 3,8 E14 Example 9

[0081] The test plate from 3 a was left in the measuring cell for the through-resistance measurement and the through-resistance (at 1 kV / mm and 90 °C) was measured according to the times specified in Table 9. Table 9: Duration of electrothermal storage and through-resistances (measured at 1 kV / mm and 90 °C) Storage duration (minutes) Contact resistance in ohms / cm 10000 (see example 3a) 8,2 E14 14400 8,7 E14 18750 8,9 E14 30150 8,8 E14 Table 10: Composition of the examples according to the invention Example Silicone polymer [wt.%] Silica [wt.%] Peroxide [wt.%] 2a 72,1 27,3 0, 6 2b 71,0 27,0 2,0 2c 70, 4 26,7 2, 9 3a 71,0 27,0 2,0 3b 71, 6 26,4 1, 9 3c 71,0 27,0 1,0 4a 64,3 33,7 2,0 4b 81, 6 16,5 1, 9 4c 98, 0 0 2,0 4d 71,0 27,0 2,0 5 71, 4 26,6 1, 9 6 97,3 0 0,7 7a, b, c, d 71,0 27,0 2,0 8 and 9 71,0 27,0 2,0

[0082] For selected examples, test plates were manufactured according to the conditions specified in the examples to produce mechanical test specimens. The measurement results are summarized in Table 11. The mechanical properties were determined using standard measurement methods. Table 11: Mechanical properties for selected examples Shore A hardness Tensile strength [N / mm²] Elongation at break [%] Tear resistance [N / mm] 2a 35 8,3 550 18 2b 37 8, 4 500 18 2c 38 9, 0 500 17 3a 38 8,5 530 18 4d 22 6, 8 630 17 7d 37 8, 4 500 18

Claims

1. A crosslinked silicone elastomer having a volume resistance adjusted to the volume resistance of an adjacent cable insulation, this volume resistance being determined on a crosslinked silicone elastomer 0.5 mm thick, in a heatable guard ring arrangement with an electrical field strength of 1 kV / mm, in accordance with standard IEC 62631-3-1 and meeting the following value after application of the test voltage: - after 10 000 minutes: < 1.0 1016 ohm*cm, obtainable by crosslinking a base composition containing: (A) 50% to 99% by weight of at least one diorganopolysiloxane having at least 2 crosslinkable groups per molecule, (B) 0.5% to 5% by weight of at least one peroxide, (C) 0% to 50% by weight of at least one reinforcing filler, and (X) NO conductive or semiconductive additives, the amount of all the components adding up to 100% by weight, wherein this base composition - is applied to a substrate or filled into a mold, - in a first step, crosslinking takes place by heating to at least the temperature of the 10 h HLT (= 10 hour half-life temperature) of the peroxide (B), the heating duration corresponding to at least 0.2 of an HL (= half-life) of the peroxide (B) at the chosen crosslinking temperature, - in a second step, heat treatment takes place above the temperature of the 10 h HLT of the peroxide (B), the heat treatment duration corresponding to at least one HL of the peroxide (B) at the chosen heat treatment temperature.

2. The crosslinked silicone elastomer as claimed in claim 1, characterized in that the heating duration of the crosslinking in the first step corresponds to at least one HL of the peroxide (B) at the chosen crosslinking temperature.

3. The crosslinked silicone elastomer as claimed in claim 1, characterized in that the heating duration of the crosslinking in the first step corresponds to at least two HLs of the peroxide (B) at the chosen crosslinking temperature.

4. The crosslinked silicone elastomer as claimed in any of claims 1 to 3, characterized in that the crosslinking in the first step takes place at a temperature of at least the 10 h HLT to at most the 1 min HLT (= 1 minute half-life temperature) of the peroxide (B).

5. The crosslinked silicone elastomer as claimed in any of claims 1 to 3, characterized in that the crosslinking in the first step takes place at a temperature of at least the 10 h HLT to at most 10°C below the 1 min HLT of the peroxide (B).

6. The crosslinked silicone elastomer as claimed in any of claims 1 to 5, characterized in that the base composition contains as (C) 15% to 45% by weight of at least one fumed or precipitated silica having BET surface areas of at least 50 m2 / g.

7. The crosslinked silicone elastomer as claimed in any of claims 1 to 6, characterized in that there are 1% to 4% by weight of at least one peroxide (B) included.

8. The crosslinked silicone elastomer as claimed in any of claims 1 to 7, characterized in that a vinyl-containing diorganopolysiloxane is used as component (A) and a vinyl-specific peroxide as component (B).

9. The crosslinked silicone elastomer as claimed in any of claims 1 to 8, wherein the volume resistance is - after 10 000 minutes: < 8.0 1015 ohm*cm.

10. The crosslinked silicone elastomer as claimed in any of claims 1 to 8, wherein the volume resistance is - after 10 000 minutes: < 6.0 1015 ohm*cm.

11. A method for producing crosslinked silicone elastomers having a volume resistance which is adjusted to the volume resistance of an adjacent cable insulation, this volume resistance being determined on a crosslinked silicone elastomer 0.5 mm thick, in a heatable guard ring arrangement with an electrical field strength of 1 kV / mm, in accordance with standard IEC 62631-3-1 and meeting the following value after application of the test voltage: - after 10 000 minutes: < 1.0 1016 ohm*cm, obtainable by crosslinking a base composition containing: (A) 50% to 99% by weight of at least one diorganopolysiloxane having at least 2 crosslinkable groups per molecule, (B) 0.5% to 5% by weight of at least one peroxide, (C) 0% to 50% by weight of at least one reinforcing filler, and (X) NO conductive or semiconductive additives, the amount of all the components adding up to 100% by weight, wherein this base composition - is applied to a substrate or filled into a mold, - in a first step, crosslinking takes place by heating to at least the temperature of the 10 h HLT (= 10 hour half-life temperature) of the peroxide (B), the heating duration corresponding to at least 0.2 of an HL (= half-life) of the peroxide (B) at the chosen crosslinking temperature, - in a second step, heat treatment takes place above the temperature of the 10 h HLT of the peroxide (B), the heat treatment duration corresponding to at least one HL of the peroxide (B) at the chosen heat treatment temperature.

12. The use of crosslinked silicone elastomers as claimed in claim 1 to 10 for insulation applications.

13. The use as claimed in claim 12 for HVDC applications.

14. The use as claimed in claim 12 for HVDC fittings.